Fuel

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To keep your E30 moving, it needs fuel. Fuel + Air + Spark = Power.

Since the majority of E30s use petrol injection engines, "Fuel" here is a synonym for "Petrol".

For Diesel engines, jump to the Diesel section of this page or learn more about Diesel.

This article primarily focuses on Motronic fuelling. For earlier fuel systems, learn more about Jetronic.

Contents

Overview

The fuel system on E30 engines M20, M40, M42 and S14 is a petrol injection system. Pressurised fuel is delivered from the tank to injectors, which squirt fuel into each cylinder in turn, at times controlled by the ECU. It is then ignited by the ignition system, and turned into lovely lovely horsepower.

The M10 engine was also available in a fuel-injected version, but the base model 316 used a more traditional carburettor setup, with ignition controlled by a mechanical distributor.

Components

Fuel Tank

63L Fuel Tank
63L Fuel Tank

All E30s have their fuel tanks mounted under the rear seat bench. However, a number of tanks were fitted to the range depending on age, engine size and body style.

The standard E30 tank is a 55L unit, with the fuel pump and a level sender screwed into the top and accessible under the rear seats. For Tourings and facelift M20-engined cars a larger 63L tank was fitted. These two tanks are interchangeable.

The carbureted 316 does not have its fuel pump in the tank. Because of that, fuel tanks for that car don't have a swirl chamber, which is basically a cup inside the tank to keep the pump submerged in fuel at all times. If you try to fit a fuel pump into these tanks as part of a tank swap or engine upgrade, you may experience fuel starvation on sharp left-hand corners.

Identifying which tank you have is relatively simple. Because of the low placement of the fuel tank into the floor pan, a central channel was required to allow the prop shaft to travel under the car; this channel effectively separates the tank into two halves. The 55L tank has a pipe connecting the two halves together at the bottom, so the level in each side is always the same, except after long corners, and so is only fitted with a single level sensor.

The 63 litre tank has a much more sophisticated internal syphon system to transfer fuel from the left side to the right, which results in the levels in the two halves not being the same. For this reason, the 63 litre tank has a sensor in each half, connected in series, to give an average level for the fuel gauge. If fitting this larger tank to a standard car, it must be paired with the appropriate fuel gauge in the dash cluster and will also need a return fuel line fitted.

Learn more about replacing the fuel tank.

Filler Pipe

The primary source of fuel delivery into your tank. The filler cap on the body connects to a stub pipe on the tank itself via a large rubber hose, which acts as an insulator. Unfortunately the seams around this stub pipe can rust, causing total tank failure.

To check the filler pipe for leaks, lift the rear o/s wheel and slide under the car. Locate the filler pipe coming from the rear wing. Grab the joining rubber pipe to the tank stub and pull it downwards.

Either the rubber pipe will pull off or fall apart, or you will see the filler stub on the tank move on it's brazed join to the tank. The rubber pipe can be replaced cheaply, but if the filler stub moves then the entire tank will have to be replaced.

Fuel Pump

Fuel pump with sender
Fuel pump with sender

The fuel pump is a relatively simple construction comprised of three components; the pump housing, the pump itself, and a fuel-level sender. The pump is a 12V electric unit by Bosch.

Because of the two types of tank, there are two types of pump housing. A 55L pump housing will have two pipes on it, while a 63L pump housing will have only one.

A two-pipe housing can be used on a 63 litre tank, provided the second pipe is blanked off, and the pipe that was connected to it re routed to a stub pipe on the side of the 63 litre tank. If this pipe is not re-routed, you will only be able to use the fuel in the RH side.

A single pipe housing cannot be used in a 55 litre tank, as there will be nowhere to connect the return line from the fuel rail, although the basic pump can be swapped between the two types.

The actual pump is rigidly fixed to the housing, and its electrical connections soldered in place. It also has a detachable filter mounted to its base. This pump is interchangeable between housings.

The level sender is held to the fuel pump housing by four screws, and must be removed before attempting to remove the pump housing from the tank. Learn more about replacing the fuel pump.

For the lowly [316i] carburreted model, fuel pressure comes from a mechanical membrane pump driven by the camshaft. It is located over the inlet manifold over cylinder n°1, held in place by two nuts, and throws an output in between 0,1 an 0,3 bar on idle.

Fuel Level Senders

Secondary level sender
Secondary level sender

While all pumps are equipped with a level sender, 63L tanks contain a second level sender on the left side of the tank, wired in series with the first. The construction of each sender is a basic float, which bridges two lengths of resistive wire. When the float is at the top, the wires are bridged close to their terminals, and thus read a low resistance. As the fuel level drops, so does the float, moving the bridge down the wires and thus increasing the resistance. In this way, each level sender works as a 60 Ohm resistor, reading zero Ohms for a full tank and 60 Ohms when empty.

Therefore the gauge for a 55 litre tank needs to read empty when in series with 60 ohms, but for a 63 litre tank gauge, it must read zero when in series with 120 ohms. Both need to read full when in series with zero ohms. This is why there are two types of fuel gauge available for the E30.

A 63 litre tank can be used with just a single level sensor, but the indicated level will vary, depending on the respective levels in the two halves.

If your fuel gauge is flickering, a broken sender is a common culprit. To check, pull the plug off them, one at a time, and short together the relevant pins in the plug, with less than half a tank of fuel on board. When the faulty one is shorted, the gauge will give an inaccurate but steady reading. Conversely, when you short the good one you will get an inaccurate and still jumpy reading.

If the gauge is still jumping around with both senders shorted out, then the brass nut on the back of the gauge, or the wiring (unlikely), is at fault.

Fuel Filter

As fuel leaves the tank, it passes through a standard canister fuel filter. The filter is mounted to the car body with a simple jubilee clip, and connects to the fuel lines using two lengths of fuel hose, again joined with jubilee clips. It is a service item, and should be replaced regularly.

Fuel Lines

Two steel brake lines run from the fuel tank to the engine bay, delivering and returning the flow of fuel. These fuel lines are attached to the floorpan of the car, and run the length of the sill, terminating at the bulkhead just underneath the brake servo. From here they connect to rubber fuel hoses.

Fuel Hoses

The fuel hoses connect the fuel lines to the main fuelling system of the engine. They are made of 8x13mm rubber, and run below the air intake in the engine bay. They are the common source of all leaks in the fuel system, and must only be replaced with genuine BMW fuel hose.

Fuel Rail

The supply hose delivers fuel to the fuel rail; a metal tube into which are screwed the fuel injectors. It is basically a pressurised reservoir, ensuring that all the injectors a given a steady supply of fuel at a constant pressure.

Injectors

The injectors are the exit point of the fuelling system. They are electrically-operated solenoids which, when activated, squirt fuel at such high pressures that it atomises into a fine mist, which is the most efficient form for it to burn.

The firing of the the injectors is controlled by the ECU, and receive their power through the DME relay. The injector power wire is red/white.

The injectors are considered one of the most important components in a well-running engine, and one of the best ways to get more power from your old engine is to get the injectors back into good working order by cleaning them, and fitting new pintle caps if necessary. Learn more about cleaning your injectors.

Injectors do not need to be upgraded, and there is no benefit in fitting bigger injectors to a standard E30 engine. Many people believe that M50 injectors are an upgrade; they can be, but only if the rest of the engine has been substantially modified already and remapped accordingly. Fitting bigger injectors alone will only result in rich running, causing damage to your engine in the long run.

Fuel Pressure Regulator

To keep the injectors squirting as they should, a constant pressure needs to be maintained which is above the pressure inside the engine. If the pressure was lower, then fuel would be blown back down the pipes to the tank when the injectors opened, so the Fuel Pressure Regulator (FPR) self-regulates the fuel pressure in comparison to the manifold.

The FPR is mounted to the end of the fuel rail, closest to the radiator. It has a small rubber tube connecting it to the top of the intake manifold, and also connects to the returning fuel hose. It is a simple diaphragm valve, sucked and blown by the intake manifold pressure to open or close the fuel flow. Failure of this diaphragm can result in rich running, lumpy idles, poor fuel economy and even engine failure. Despite this, the FPR does not have a reputation for unreliability.

Carburettor

The lowliest model, the 316, was kitted out with the same carburettor technology that had seen service in the E21, starting with the Pierburg 2B4 carburettor before "upgrading" to the Ecotronic 2BE in 1984. These later carbs make use of a Bosch electronic control module, nr 0 260 200 002.

While massively advanced at the time, these carbs have since proved to be unreliable to the point of hateful, and almost any fuelling the 316 suffers from can be traced back to the Pierburg unit.

A proven solution is to replace the entire setup with an equivalent carburettor from Weber. Learn more about Weber carburettors.

Carbon Canister

To meet increasingly stringent emissions regulations, later engines were fitted with a Carbon Canister. This small brown container sits in the engine bay and is bolted to the inner wing. It captures fumes from the fuel tank venting system, filtering them through a self-contained carbon filter and releases them into the throttle body. The carbon canister was controlled by a small electric valve. If you find an unused plug on the loom underneath the throttle body, it is for this valve.

Utterly redundant, the Carbon Canister system causes more problems that it solves in older cars, and is recommended to be removed if found, or at least disabled.

Removal is simple. When disconnecting, the end of the rubber hose to the canister from the back of the car MUST be left open, and should be tucked inside the reinforcement for the suspension turret. The hose from the valve that goes towards the inlet manifold MUST be blocked off and airtight, either by a plug in the end of the hose, or a cap on the metal stub pipe into the manifold.

For cars not fitted with the Carbon Canister, it is this blanking plug on the intake that is a major cause of air leaks.

Controlling

The injection system is the main function of the ECU, be it L-Jetronic or Motronic. The ECU reads data on the engine from the Crank Position Sensor, and activates the fuel pump via the Fuel Pump Relay. This provides power directly to the fuel pump through Fuse 11. It also activated the DME relay, which powers the injectors and the ICV.

Fuel Pump Relay

The Fuel Pump Relay is located in the engine bay, under a plastic relay holder with space for three relays only. On M20 engines, the relay holder is mounted next to the passenger side suspension turret. On M40 and M42 engines, the relay holder is mounted to the bulkhead next to the brake servo. The fuel pump relay SHOULD occupy the middle position; to confirm, check that the relay has a green/violet wire feeding it.


Common Problems

Before attempting ANY work on the fuel system, it is important to de-pressurise the system by removing Fuse 11 and cranking the engine until it refuses to fire. If you are trying to diagnose fuel problems, you must still de-pressurise the system first.

No Fuel to Engine

If your engine is not starting, it is important to ascertain whether the engine is receiving both fuel and spark. Make sure you have tested for spark before you proceed further. If you are not receiving spark or fuel, check the Crank Position Sensor.

If spark is definitely present, the test to see if any fuel is reaching your engine is to detach one of the hoses in the engine bay, put the end into a container (such as a water bottle) and then crank the engine. If fuel gets to the bottle, you have fuel. To do this:

  • De-pressurise the system by removing Fuse 11 and cranking the engine
  • Locate the thick rubber hoses leading to the FPR or fuel rail
  • Undo the jubilee clip to the nearest accessible hose, pull the hose off and put the end into a securely-placed container
  • Replace Fuse 11
  • Crank the engine for a few seconds
  • Check the bottle

If fuel is present in the bottle, your fuel system is working; however, your injectors may not be delivering the fuel to the cylinders. If this is the case, check your DME relay.

If fuel is NOT present, elements of the system must be eliminated to identify the cause. Follow these steps to troubleshoot the system:

  • Check Fuse 11
  • Identify and remove the Fuel Pump Relay.
  • With the relay removed, bridge pins 30 (red wire) and 87 (green/violet) to directly power the fuel pump. Turn on ignition, and check for fuel.
  • If fuel is now present, the fault lies in the controlling system.
  • If no fuel is present, remove the fuel filter and place a suitable fluid containter here. Crank engine. If fuel is now present, replace your fuel filter.
  • If fuel is still not present, the fault lies with your fuel pump.

Fuel Smell (Inside)

A strong smell of fuel inside the vehicle is normally attributed to leaks around the fuel pump housing. The simplest cause of this is a perished rubber O-ring that fits between the pump housing and the tank, and is replaced easily. The second cause will be the pipes connecting to the pump housing, and the braided rubber vent pipe connected to the tank. The ends of these pipes perish and split over time, but usually an inch can be cut from the end of these pipes and still leave enough to re-attach to the tank.

A more serious cause of the smell would be rust of the tank itself, most notably at the joint with the filler pipe. Unfortunately this necessitates a removal of the tank, and a repair or replacement. Learn more about removing the fuel tank.

Fuel Smell (Outside)

A fuel smell outside, either around the engine bay or the rear wheel arches, is indicative of a leak in that area.

Fuel Leak

Because of the intricate pipework, indentifying the location of a leak can be tricky. You could just wave a cigarette lighter around until you find it, but there are better ways. In each location of the car there is a usual suspect:

Fuel Leaks
Leak Location Cause Solution
Engine Bay Perished fuel hoses Replace hoses
Rear Passenger Arch Fuel Filter Replace
Rear Driver Arch Rusty filler pipe Grab the rubber joining pipe and pull. If the stub pipe moves, replace fuel tank.

Rich Running

Rich running is diagnosed as a strong fuel smell from the exhaust, poor throttle response and even stalling at idle. The richness of the system is controlled by the Motronic system, and must not be adjusted without the proper equipment (such as an exhaust gas analyser).

If your system is running rich, the most likely problem is the Blue Plug.

However, if your Blue Plug proves not to be the problem, the next step is the wiring plug C191, which connects the temperature sensors to the main loom (as well as the injectors.) It is located under the inlet manifold. On M20 engines this plug collects moisture and corrodes, rotting the wires and causing electrical issues such as rich running.

On a few M20 engines, an official BMW bodge was made to create a cold start relay, which can break with time.

Flooding

Flooding is when too much fuel enters the cylinders, making the spark plugs too damp to fire. Flooding can be caused by an engine running too rich, or by leaking and dribbling injectors.

To start a car that has been flooded, remove the Fuel Pump Relay and crank the engine. The engine will try to fire and stumble a few times, before cranking normally with no firing. At this point, re-insert the Fuel Pump Relay.

Cold Start

The E30 fuel system is designed to be constantly under pressure, which means that fuel should be pumped straight into the engine the moment the key is turned. If this doesn't seem to be happening, it's very likely that the system is losing pressure, which indicates a leak. Check all of your lines and hoses, especially with the engine running, and look for a mist or spray. Fix any leak immediately.

The is also the chance that your injectors need to be reconditioned. As injectors age, they wear and do not form a perfect seal when closed. This allows fuel to dribble out and into the cylinder when the engine is shut off, de-pressurising the system. Get them restored.

If the engine still refuses to start when cold, the problem is unlikely to be fuel-related.

Hot Start

Hot start (or warm start issues) are issues related to the engine running too rich.

Upgrades

None at this time.

Diesel

Main article: Diesel

Diesel was not a popular fuel, and remains more expensive per litre in the UK than in other countries. In continental Europe however, the M21 engine powered two diesel versions of the E30; the 324d and the turbo-charged 324td.

Although a small number of these cars were imported to the UK, they remain very rare and parts are very hard to obtain. If you are considering a diesel for reasons of fuel economy, it is highly recommended to consider installing LPG instead, due to the comparative simplicity of the system compared to a diesel conversion.